The present invention relates to a linear multichannel detection system for billing by spectrum convolution applicable in telecommunications systems. More specifically, the invention relates to a system capable of detecting metering pulses in a signal sent from a telephone exchange to the subscribers, multiplying said signal by a specific signal generated by said system.
In developing present day telecommunications systems, when implementing systems for connecting to typical telephone exchanges, for example of the type referred to as POTS or xe2x80x9cPlain Old Telephone Servicexe2x80x9d, the problem arises of detecting billing signals, more commonly referred to as xe2x80x9cmeteringxe2x80x9d. These signals are transmitted by the telephone exchange to the subscriber line, and carry information typically related to financial charges to be paid by the subscriber, for the provision of the telephone service.
These signals take the form of pulses that are usually at high frequency, i.e. between 12 and 16 kHz, transmitted transversally across the two wires of the telephone line. The billing function has to be implemented on a basis of detecting these pulses on the subscriber side.
These pulses, however, can appear at any moment in the communication, for example during the telephone conversation. This means that the detector employed for detecting said pulses must be capable of perfectly suppressing the audio band in order to be able to discriminate the signal to be detected. Likewise, high frequency noise, for example at 20 to 60 kHz and having an amplitude of several volts, can be present on the line, despite which the pulse detecting system has to detect these and only these pulses, said pulses having typically an amplitude of a few times ten millivolts.
These severe detection requirements, translate in practice into really tight discrimination specifications which are published for each country in bodies like the ETSI, and which, in a word, lead to the requirement for using linear bandpass 2 to 10-order filters, depending on the country, for each subscriber line.
The current solutions are based on implementing these filters, subscriber by subscriber. The technologies employed are normally the following:
1. Passive filters, comprising standard L-C networks with inductor setting by adjustable air gap.
2. Active filters, comprising R-C/Operational Amplifier networks, with setting by potentiometer.
3. Active ceramic filters with precision resistors trimmed by LASER.
4. Switched capacitor filters, with precision resistors and ceramic substrate.
A drawback of this type of solution is that to implement a high order filter, for example up to 10, for each subscriber, a complex production system has consequently to be set up. For example, in the case of using passive components, it is necessary 5 inductors and 5 capacitors, with a manual procedure for inductor setting. If this solution is employed with interface boards in a system which is connected to a telephone exchange, and which provides service for a significant number of subscribers, for example up to 1024 subscribers, this solution can imply a high cost in:
1. Components: 5 inductors and 5 capacitors, precision type, 0.67%.
2. Component setting: manual setting of inductors, potentiometers, or ceramic resistors, with LASER.
3. Space on the printed circuit board.
4. Hardware inflexibility in the event of changes in specification among different countries, which at times can require a different detection mask.
Consequently it has become necessary to provide a system in which the detection of metering pulses is achieved in a more economical and less complex fashion.
The present invention proposes a system for detecting at least one given frequency within a received signal, said given frequency belonging to a particular frequency band, which is characterised in that it comprises means for generating at least one specific signal, the spectrum of which corresponds to said particular frequency band; means for multiplying said received signal by said at least one specific signal; and means for measuring the corresponding powers of the signals resulting from said multiplication, so that a measured power greater than a threshold value, indicates that said given frequency has been detected.
According to a preferred embodiment of the invention, the system is characterised in that the specific signal is broken down by analysis into two signals, these being the in-phase and quadrature components of the specific signal having the same spectrum as the latter, for the purpose of performing amplitude detection of the incoming signal, independently of its phase.
According to another preferred embodiment of the invention, the system is characterised in that said means for generating at least one specific signal comprises a digital unit for signal spectrum encoding capable of generating digital signals.
According to another preferred embodiment of the invention, the system is characterised in that said digital signals contain spectral specifications of the network to which the subscriber is connected, in the form of digital encoding of a mask for detection and rejection of frequency levels.
According to another preferred embodiment of the invention, the system is characterised in that the signal resulting from the multiplication is applied to a filter, preferably low-pass, the output from which is squared for providing an estimate of the power spectral density of the specific signal, scaled in proportion to the amplitude of the signal received, so that said spectrum behaves substantially the same as the desired detection mask.
Likewise, the present invention proposes an access system incorporating the detection system of the invention.
The solution proposed by the invention drastically reduces the costs mentioned above since it significantly reduces the space occupied on the printed circuit board; it does not require any manual setting process of components since the boards can be produced with a perfectly valid detection mask; it does not therefore require precision technology, or a high cost as in the case of filters developed on a ceramic substrate, with LASER-trimmed precision resistors. In addition, the change from one detection mask to another due to the different specifications of each country, proves very straightforward for an expert in the matter as it would be a question of changing the programming in a memory device (for example, an EPROM), common for all subscribers to the system.